Necrotizing enterocolitis (NEC) in preterm infantsjs.a complex, multi-factorial disease typically characterized by an exaggerated inflammatory response to pathogenic flora leading to bowel necrosis and often death. Dysregulated intestinal immune development may play an important role in the disease process, though this area is still poorly defined. To advance our understanding of NEC pathophysiology we propose to isolate adaptive immune cells from the lamina propria of surgically resected tissue from NEC patients and controls. We will test the hypothesis that a relative absence or reduced function of lamina propria F0XP3^ T regulatory cells (Treg) in the ileum of predisposed preterm infants is a risk factor for NEC. Treg are professional immune suppressor cells that are typically characterized as interleukin-2 receptor a (IL-2Ra = CD25)''3''' CD4^ CD45RO* IL-7R (CD127)*' T cells expressing high levels of the transcription factor forkhead box protein 3 (FOXPS). Depletion of Treg from the normal CD4* T cell pool or mutation ofthe FOXPS gene results in immune dysregulation involving the intestinal tract. The Specific Aims ofthis proposal are to test the following three hypotheses: (1) Intestinal T effector cells (Teff) from preterm infants with NEC are less susceptible to Treg function. Intestinal Teff from preterm infants with NEC and controls will be compared for responsiveness to Treg by measuring inhibition of T cell proliferation. IL-2, IL-6, IL-17, TNF, and TGF-p levels in supernatants of Teff cultures will determine if NEC patients provide factors that promote or inhibit Treg generation, (2) Treg function in the ileum of preterm infants with NEC is limited. We will measure if Treg presence and/or function is reduced in NEC patients by comparing T cell suppression, cytokine production and FOXPS mRNA levels between NEC patients and controls and (3) Cord blood from preterm infants with NEC contains lower percentages of gut-homing Treg compared to controls. We will collect cord blood from infants less than 35 weeks gestational age and compare Treg presence and function between NEC patients and controls. Our findings will provide a more robust understanding of the immunological mechanisms that regulate the intestinal development of premature infants. This proposal develops novel translational approaches to identify cellular and molecular targets for reducing the pathologic host immune response in NEC and to detect infants at highest risk of this devastating disease.